Cathode-ray tube



Feb. 23, 1943 H RG Re. 22,275

CATHODE-RAY TUBE Original Fild July 25. 1940 O. I'ERGENROTHER DOLF ATTORNEY Reissued Feb. 23, 1943 R UNITED STATESPATENT ()FFICE CATHODE-RAY TUBE Rudolf C. Hergenrother, Beechhurst, N. Y., as-

signor to Hazeltine Corporation, a corporation of Delaware Original No. 2,295,038, dated September a, 1942, Serial No. 347,389, July 25, 1940. Application for reissue December 21, 1942, Serial No.

18 Claims.

- The present invention relates to cathode-ray tubes and, particularly, to an improved cathoderay tube especially suitable for use as the image reproducer in television systems.

within certain limits because the spherical aberration of the second electron lens increases as the square of the effective lens aperture for any given lens. This aberration must be kept below Conventional cathode-ray tubes include a the value at which it seriously affects the spot multi-electrode electron gun, the function of size. Aside from spot size, the shape of the which is to produce a beam of electrons of eleg spot on the fluorescent screen tends to be dismentary cross-section. The electron gun of torted by the deflecting fields of the scanning cathode-ray tubes employed in television systems system. This distortion increases as the diamgenerally has as an element thereof a grid which eter of the cathode-ray beam in the scanning is used to modulate the electron beam in accord- Z n and t r y increases directly With the ance with video signals applied thereto. The angle of divergence. The spot size at the center first anode, grid, and cathode elements, when of the fluorescent screen is thus determined by suitably energized, form a first electron lens of the combined effects of cross-over size and the tube which serves to gather electrons emitted 5 sp e a aberration of the Second electron lens. by the cathode and. concentrate the electrons W reas t Spo e at e edges o e fl eswithln a beam of small cross-sectional area adcent screen is additionally afie t d by the distorjacent the grid aperture. There is thus formed tion pr y the deflecting fields 0f the a constriction in the cathode-ray beam known Sca system. The scanning disto m y as the electron cross-over which serves as the be qu Serious in Cathode-my tubes adapted to electron object to be focused on the fluorescent have the atho -ray b am flec e ov r a Wide screen of the tube by a second electron lens of angle and ay determine the pp t n t e either the electrostatic or magnetic type. The permissible beam current i these tubessize of the electron-cross-over, and hence the As an aid in reducing both the Space Charge, size of the focused electron spot on the fluoresand Consequently the size of the Cross-Over, and cent screen, depends on the focal properties, that the angle of divergence of the electron a f r is, on the electric field distribution in the first y given luminous intensity of the p i h electron lens. been found .desirable to produce the maximum It has been shown that, in th ab of electrostatic field intensity close to the grid aperspace-charge effects, the size o1" the electron ture of the first electron lens. This has been accross-over for a given elect n gun t given complished in certain tubes of the prior art by anode potentials depends only on the energy disforming the first anode as an p rtu frustumtribution of the thermionic electrons leaving the shaped member of small diameter and y p cathode. In practice, however; the size of the sitiening the apex of the anode in Close Proximity electron spot on the fluorescent screen increases d coaxially aligned W the gridap rquite rapidly with increasing cathode-ray curtllre- In t arrangement, the spacing f th rent, due, at least partly, to the effect of increasanode from the grid and the size of t n d ing space charge at the cross-over hi h efaperture are the only critical factors in reduc fectively increases the cross-over size. It is, 5 the magnitude 0f e Sp c C a e and the therefore. desirable to decrease the space charge angle of divergence o e e e o beam. Such to a minimum, thus increasing th maximum tube construction has the disadvantage first, that permissible value of the cathode-ray current the alignment of the anode and grid is eXtlemely which is limited to a value corresponding to the Critical for setisfeetmy p at o o e tu e maximum spot size which will still ermit full and, secondly, the first anode receives electrons resolution of image detail. Any decrease in ly r h cathode. h y not y cross-over size is very desirable since it will allow Wasting power but Causing the anode to eat p an increase in the picture brightness without to a high temperature and also T ss he loss of detail, or will allow a decrease in the cathode-ray beam by secondary electron emisrequired value of the anode voltage at the same 51011 from the anodepicture brightness without loss of detail. Other cathode-ray tubes of the prior art have In addition to the effect of space charge on attempted to reduce the space charge and dethe cross-over size, increasing values of beam crease the angle of divergence of the electron current cause the angle of divergence of the elecbeam by the use of an apertured, disc-shaped, tron beam leaving the electron cross-over to infirst anode closely spaced to, and coaxially crease. The angle of divergence must be kept as; aligned wi h, the grid. Arr n f hi wires and the electrical connection to the outside of the bulb may produce cold electron emission or electrical breakdown of the insulation when high anode voltages are used.

It has also been proposed that the size and distortion of the luminescent spot on the fluorescent screen be reduced for a. iven image detail and brightness by positioning a cylindrical first anode of diameter only slightly larger than that of the grid in coaxial relation to the grid with the proximate ends or the anode and grid lying substantially in the same plane. Tubes of this nature have the disadvantage thatthey require an excessively high second anode potential and require a relatively complicated and expensive construction by which the first anode is supported from the neck of the cathode-ray tube.

It is an object of the invention, therefore, to provide a cathode-ray tube of simplified and improved construction which, while of general application, is especially suitable for use as the image reproducer in television systems and one which avoids one or more of the above-mentioned disadvantages of the prior art cathoderay tubes.

It is a further object of the invention to provide a cathode-ray tube which requires for its proper operation a value ofsecond anode potential appreciably smaller than that required by similar tubes of the prior art, yet one in which the reduction of anode voltage does not involve any sacrifice of picture brightness nor loss of picture detail.

It is an additional object of the invention to provide a cathode-ray tube capable of forming a cathode-ray beam having a minimum size of cross-over and minimum angle of divergence, whereby, for a given picture detail and given value of anode voltage, a brighter picture is produced and there is little or no distortion of the luminescent spot as the beam is deflected during the scanning operation.

In accordance with the invention, a cathodeas 50 milliamperes per square centimeter of emissive surface.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

Referring now to the drawing, Fig. 1 is a side elevation, partly in section, of a complete cathode-ray tube embodying the invention in a preferred form; Fig. 2 is an enlarged cross-sectional view of the electron gun of the Fig. 1 arrangement; and Fig. 3 represents a modified form of grid and cathode assembly suitable for use in the cathode-ray tube of the invention.

It has previously been pointed out that the spot size characteristic can be improved by reray tube comprises a source of electrons, a first electron lens comprising an annular first anode and an annular apertured control electrode surrounding said source of electrons, and means for supporting the control electrode and the anode substantially in coaxial alignment. The control electrode has an external dimension transverse its axis less than 0.15 times that of 2. corresponding internal dimension of the anode measured transverse the axis thereof.

In a specific form of the invention, a cathoderay tube comprises an evacuated tube envelope having a substantially cylindrical neck of uniform cross-section along its length, and a first electron lens comprising a substantially cylindrical grid and an electrically conductive first anode coating on the Walls of the neck of the tube envelope. The grid and cathode are preferably as small as manufacturing facilities ermit, the cathode having an emissive surface, the permissible electron emission of which is at least as high duction both of the space charge, and consequent reduction of the cross-over area, and angle of divergence of the electron beam. This may be accomplished by the expedient of increasing in the vicinity of the grid aperture the electrostatic field intensity of the first electron lens. The present invention makes use of the phenomenon that the electrostatic field intensity at the end of the smaller of two cylindrical, axially displaced, and coaxially arranged conductors of different diameters is rapidly increased as the diameter of the smaller of the two conductors is reduced to an infinitely small value. In accordance with the invention, the larger cylindrical conductor is made the anode of the tube while the smaller conductor is made the grid. The latter, however, necessarily encloses the cathode, the emissive surface of which must be perpendicular to the axis of both the anode and the grid. Thus. the required electron emission for a given beam current constitutes a definite limitation on the minimum physical size of the cathode and thereby on the minimum diameter of the grid.

An oxide-coated cathode operates at electron saturation of the order of 700 milliamperes per square centimeter of emissive surface. Although operation at current saturation causes a rapid deterioration in electron emission, thereby shortening the life of the cathode-ray tube, the shortening in life is disproportionately smaller'with values of cathode current only slightly below saturation and it is thus possible to have a cathode of sufficiently long life as to be commercially practicable when the cathode is operated at an electron-emission density greater than 50 milli-- amperes per square centimeter of cathode-emissive area and, in fact, of the order of several hundred milliamperes per square centimeter of emissive area. It is obvious that the smaller the cathode-emissive area, the smaller the diameter or the effective external dimension transverse the axis of the surrounding grid may be, whereby the grid and its enclosed cathode approach an electrical conductor of relatively small dimensions with consequent high increase in the electrostatic field intensity in the vicinity of the grid aperture. The lower limit of the cathode-emissive area in square centimeters is equal to the maximum required beam current in milliamperes divided by the maximum allowable emission current density which, as has already been stated, may vary from 50 to several hundred milliamperes per square centimeter depending on the specific nature of the oxide emitter. The limit in this regard is, in general, reached when the cathode and. grid become so small as to be limited only by practical manufacturing capabilities.

Referring now to Fig. 1 of the drawing, there is illustrated a complete cathode-ray tube embodying the invention. The tube comprises an evacuated tube envelope 10 having a substantially cylindrical neck portion II preferably of uniform cross-section along its length and a flared bulb portion l2. One end of the neck II is formed into a re-entrant stem 9 through which are sealed, and from which are supported, a plurality of relatively rigid electrical conductors l3. These conductors support a substantially cylindrical or annular apertured control electrode or grid I4 substantially in coaxial relation or alignment with respect to the tube neck II and also complete electrical circuits from the grid l4, and

from the thermionic cathode or source of electrons and its heater, which are not shown but which are enclosed within the grid, to prongs l5 provided on a tube base l6 fixedly secured to the closed end of the tube neck.

The inside walls of the neck II and the flared bulb l2 have an electrically-conductive coating l1 formed thereon to provide a substantially cylindrical or annular first anode. This coating, which may be formed by deposit from a commercial product merchandised under the trade name Aquadag, a commercial form of an aqueous colloidal suspension of graphite, is a continuous electrically-conductive anode coating which extends from a fluorescent screen l8, formed on the flared end of the tube envelope, along the inner walls of the flared bulb and neck portions of the envelope to a plane which is perpendicular to the axis of the neck II and which preferably includes the apertured end of the grid 14. The distance between the plane defining the end of the anode cylinder and the grid aperture is not critical. The apertured end of the grid may be placed far within the anode cylinder or may be withdrawn sufliciently that the distance from the grid aperture to the plane defining the end of the anode is of the order of one-half the anode radius, either extreme position of the grid having no significant change in the first electron lens action as long as the axial alignment of the grid and anode is maintained. The effect of any irregularities in the trim of the anode cylinder on the electric field at the grid aperture would increase as the grid is withdrawn from the anode and decrease as the grid is placed deeper within the anode. Electrical connection is made to the coating I! through a lead-in conductor l9 sealed through the side of the tube envelope I2. The grid 14 and anode II, when suitably energized, provide a first electron lens, the function of which is to produce a cathode-ray beam of elemental cross-section at its electron cross-over and directed toward the fluorescent screen l8 of the tube.

A second electron lens of the magnetic type is provided by a focusing coil 2|], comprising a large number of turns of wire disposed coaxially around the tube neck II and displaced axially from the first electron lens toward the screen l8. The purpose of focusing coil 20 is, as well known in the art, to reconverge or focus the cathoderay beam, which diverges beyond its cross-cver, to a relatively small spot on the fluorescent screen l8. Instead of a magnetic focusing coil 20, an electrostatic type second electron lens can be provided simply by removing a ring of the coating I! at about the mid-plane of the focusing coil 20, thereby to provide a first anode in the vicinity of the grid l4, the first anode being provided with aseparate electrical connection, and a second. anode which extends along the neck of the for supporting the grid l4 and the anode l1 subtube and into the flared bulb portion, these two anodes being suitably energized to accomplish the required focusing of the cathode-ray beam on the fluorescent screen It.

The position of the scanning coils or plates along the neck of the cathode-ray tube is generally indicated at 2|. These scanning coils or plates operate in well-known fashion to deflect the cathode-ray beam in a predetermined pattern of scanning lines on the fluorescent screen of the tube.

Fig. 2 is an enlarged cross-sectional view of the electron gun of the tube illustrated in Fig. 1. Elements corresponding to like elements of Fig. 1 are designated by like reference characters. The grid l4 projects through and is supported in an orifice or aperture 22 in the apex of a hollow conical member 23 of insulating material, for example, steatite. The hollow conical member 23 is open at its enlarged end and comprises a means stantially in coaxial alignment. The member 23 has an annular rim 24 around the circumference of which are a plurality of apertures 25 through which the relatively rigid conductors l3 are looped. The conductors, as previously stated, support this assembly from the reentrant stem 9 of the tube with grid I 4 in axial aligmnent with the neck ll of the tube envelope. The relatively rigid electrical conductors I3, each of which extends through one of the apertures 25 in the rim of the hollow member 23, are sealed through the tube envelope II for supporting the member 23 within the tube and, in addition to rendering the aforementioned support, serve to complete electrical circuits from the grid and cathode externally of the tube.

A thermionic cathode which preferably includes an indirectly-heated substantially cylindrical cathode sleeve 26 is fixedly disposed within, and in spaced relation to, the inner walls of the grid II by. an insulating bushing 21 and spacer rings 28. One end ,of the cathode sleeve 26 is provided with an electron-emissive surface 29 in opposing relation to an aperture 30 in the end of the grid [4. Within the cathode sleeve 26 is a heater 3|.

A suitable electrically-fired getter 32 is positioned within the member 23 and near the apex thereof. Two of the conductors l3 are used to complete an electrical circuit to the getter material and comprise a means for supporting the electrically-fired getter within the member 23 and near the closed end thereof. The member 23 thus acts as a shield to prevent overheating of the getter material during seal-in of the reentrant stem of the tube and also acts as a shield when the getter is fired to prevent the getter material from condensing on the inner walls of the neck. ll near the first anode IT. The getter material is conductive and, for that reason, would impair the symmetry of the electrostatic field between the anode l1 and grid I4, thereby impairing the first electron lens action, if it were allowed to be deposited on the inner Walls of the neck I l in contact with the first anode H.

The area of the electron-emissive surface 29 is sufficiently small that the maximum required electron emission is not less than 50 milliamperes per square centimeter of emissive surface and is preferably as much higher than this value as is consistent with the required length of life of the cathode-ray tube. The grid aperture 30 is preferably made between about 0.5 and 0.75 times the diameter of the cathode-emissive surface 29, the

that of the cathode sleeve 26. The grid is preferably spaced from the electron-emissive surface 29 a distance about equal to the diameter of the grid aperture 3!]; The internal diameter of thegrid I4 is between about 1.5 to 3 times the diameter of the cathodeslee've 26. In order that there shall be developed adjacent the grid aperture 3!! an electrostatic fieldof high intensity, the external diameter or dimension transverse the axis of the grid I4 should be not greater than, but preferably less than, 0.15 times that of the internal diameter or dimension measured transverse the axis of the firstanode I I. As an aid in attaining the desired high intensity field gradient, the closed end of the grid I4 is rounded to have a substantially hemispherical contour.

It will be evident from Fig. 2 that the end of the first anode II lies in a plane perpendicular to the axis of the neck II at substantially the apertured 'end of the grid I4. It is this arrangement of the substantially cylindrical first anode I'I, substantially cylindrical apertured grid I4, and cathode 26, 29 which comprises the first electron lens of the tube.

Fig. 3 represents a modified grid configuration suitable for use in a tube constructed in accordan'ce with the invention. One end of the grid I4 is closed by an apertured cup-shaped cap 33 whereby the closed end of the grid is flat or blunt. Similarly, the cathode sleeve 26 is closed at one end by a cap 34 upon the bottom of which is formed the electronemissive area 29. The cathode sleeve is maintained coaxially aligned within the grid I4 by insulating discs 35, 35 which are held in position by split rings 36,31, and 38. The grid I4 is provided with a slot 39 to permit the closed chamber 40 thus formed to be suitably exhausted when the tube itself is exhausted during the forming period. This grid construction is slightly more complex than that of the Fig. 2 construction, but has substantially similar characteristics and is much better from the standpoint of mechanical construction. Additionally, the construction permits more accurate alignment of the cathode and grid.

It will now be apparent from the foregoing description of the invention that the invention has a number of outstanding advantages in addition to the improvement of the spot characteristic. The size of the oxide-coated electron-emissive area of the cathode has been greatly reduced, thus reducing the amount of material which is released from the cathode during the breakdown or forming process, whereby the loss of emission during the operating life of the tube by negative ion bombardment of the emissive surface is correspondingly reduced. Similarly, the amount of metal in the electron gun structure is reduced, thus minimizing the amount of gas which must be released by bombardment during the forming process and which is released during normal operation of the tube.

A tube constructed in accordance with the invention has the advantage of simplicity of construction, the first anode being merely a coating on the inner wall of the neck of the tube envelope, or being a substantially cylindrical conductor closely fitting the inner walls of the neck and supported directly therefrom. Where the anode is comprised by a coating, the problem of properly trimming the end of the anode adjacent the grid is not critical, since almost the entire first electron lens action is localized in the vicinity of the grid aperture. Furthermore, the problem of diameter of the surface 29 being substantially properly aligning the first anode and grid is also less critical since the size of the grid is decreased to the order of the grid aperture size and the coaxial relation is not critical, also because of the localization of electron lens action.

The decrease in the area of the electron-emissive surface of the cathode results in a substantial reduction of the heater wattage. Whereas the prior art cathode-ray tubes generally require about 5 watts of cathode heater power, a tube constructed in accordance with the invention requires only about 0.8 watt. This means not only reduced power drain from the heater supply, but also a decrease in the temperature of the metal parts of the tube, a decrease in the temperature of the glass wall of the tube neck and the reentrant stem and its seal, and a decrease in the temperature of the getter of the tube whereby there is less tendency of all of these elements to 1evglve gas throughout the operating life of the An additional advantage of the invention is that the grid-to-cathode capacitance is material- 1y decreased by the relatively small dimensions of the electron gun elements. Thus, while tubes of the prior art generally have a grid-cathode capacitance of fromlO-20 micro-microfarads, a tube constructed in accordance with the invention has a grid-cathode capacitance in the order of one or two micro-farads.

While the grid and the first anode have been described as cylindrical or substantially'cylindrical, it is to be understood that these terms are to be construed according to their generally accepted usage to define and describe hollow generally cylindrical or annular elements or elements of a configuration which are equivalent thereto insofar as they exhibit the aforementioned electrostatic field intensity phenomenon attributed to cylindrical conductors of unequal diameters.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A cathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a substantially cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, and mean for supporting said grid and anode substantially in coaxial alignment.

- 2. A cathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a substantially cylindrical first anode axially displaced from said grid, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment.

3. A cathode-ray tube comprising, a thermionic cathode having a predetermined minimum permissible size, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a substantially cylindrical first anode, the external diameter of said 75 grid being not reater than 0.15 times that of apertured grid surrounding said cathode and an electrically-conductive first anode coating on the inner walls of the neck of said envelope, said grid having an external diameter less than .l5 times that of the internal diameter of said conductive first anode coating, and means for supporting H said grid substantially in coaxial alignment with said anode coating.

5. A cathode-ray tube comprising, an'evacuated tube envelope having a substantially cylindrical neck, a thermionic cathode, a first electron lens comprising asubstantially cylindrical aper= tured grid surrounding said cathode and a cylindrical first anode closely fitting the inner walls bf said'neck, said grid'having an external diameter lessthan 0.15 times that of the internal-diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment.

6. A cathode-ray tube comprising, an evacuated tube envelope having substantially cylindrical neck and flared bulb portions, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a continuous electrically-conductive anode coating on the inner walls of the neck and flared bulb portions of said envelope, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode coating, and means for supporting said grid substantially in coaxial alignment with said anode coating.

7. A cathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a substantially cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, means for supporting said grid and anode substantially in coaxial alignment, and a second electron lens substantially coaxial with and axially displaced from said first electron lens away from said cathode.

8. A cathode-ray tube comprising, a thermionic cathode having an electron-emissive surface, the permissible electron emission of which is at least as high as 50 milliamperes per square centimeter of emissive surface, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a substantially cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment.

9. A cathode-ray tube comprising, an indirectly heated cathode having a cylindrical sleeve and an electron-emissive surface at the end thereof, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a substantially cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode and an internal diameter not less than 1.5 times that of the external diameter of said cathode sleeve, and means for supporting 'saidgrid, said cathode sleeve, and said anode substantially in coaxial alignment.

10. A cathode-ray tube comprising, a thermionic cathode having a cylindrical sleeve and an electron-emissive surface at the end thereof, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode in spaced relation thereto and a substantially cylindrical first anode, the aperture of said grid having a diameter substantially equal to the spacing of said grid and cathode and said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment.

11. A cathode-ray tube comprising, a thermionic cathode having a substantially cylindrical sleeve and an electron-emissive surface at the end thereof, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode, the aperture of said grid having a diameter substantially between 0.5 and 0.75 times the external diameter of said cathode sleeve and said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid, said cathode sleeve, and said anode substantially in coaxial alignment.

12. A cathode-ray tube comprising, an evacuated tube envelope having a cylindrical neck, a thermionic cathode having a cylindrical sleeve and an electron-emissive surface on one end thereof, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode disposed on the inner surfaces of the neck of said envelope, the electron emission of said emissive surface being not less than 50 milliamperes per square centimeter of said emissive surface and said grid having a diameter substantially between 1.5 and 3 times the diameter of said cathode sleeve and less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid, said cathode sleeve, and said anode substantially in coaxial alignment.

13. A cathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, means for supporting aid grid and anode substantially in coaxial alignment comprising a hollow member of insulating material open at one end and closed at the other end except for an orifice through which said grid projects, and means for supporting an electrically-fired getter within said member near the closed end thereof.

14. A cathode-ray tube comprising, a thermionic cathode, a first electron lens comprising a substantially cylindrical apertured grid surrounding said cathode and a cylindrical first anode, said grid having an external diameter less than 0.15 times that of the internal diameter of said anode, means for supporting said grid and anode substantially in coaxial alignment comprising a hollow conical member of insulating material having an open base end and an apertured apex through which said grid projects, and means for supporting an electrically-fired getter within said member near the apex thereof.

15. A cathode-ray tube comprising, an evacuated tube envelope, a thermionic cathode, a first a cylindrical first anode, said grid having'an external diameter less than 0.15 times that of the internal diameter of said anode, and means for supporting said grid and anode substantially in coaxial alignment comprising a hollow. conical member of insulating material open at the. base and supporting said grid and cathode at the apex thereof, and relatively rigid electrical conductors each extending through apertures in the rim'of said member and sealed through said tube en.-

ternal dimension transverse its axis less than 0.15

times that of a corresponding. internal dimension of said anode measured transverse the, axis thereof.

electron lens comprising a substantially cylindri cal apertured grid surrounding said cathode and A q h e ay be mprisi a. sour e of e t ons a s ec on e s c pr s ngn annular firstanode and. an. annular anertured control electrode surrounding said source oi electrons, and meansfor supporting said control electrode and said anode substantially in; coaxial al e d ontr l elect o e h ms; maximum external dimension transverse its; axis less than 0.15 times that of a corresponding internal dimension of said anode measured transverse'the axis thereof at the position of, closest spacing. between said anode and said aperture of said con.- trol electrode.

18. A cathode-ray tubecomprising, asource of electrons, a first electron lenscomprising. anannular first anode and an annular: apertured control electrode surrounding said. source. of; elec- V trons, and means for supporting said control elecrtrode and said, anode substantially in. coaxial alignment, said control electrode having an external diameter less than 0.15 times; that, of the internal diameter of saidqanode measured at the position of closest spacing between said anode and said aperture of said control electrode- RUDOLF C. HERGENROTHER; 

